Thread: 8" Ford rear end R&P questions

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I have a 32 Ford roadster and I need a little lower gear ratio for better acceleration. Here's some details, about 2500# weight, with a Chevy 350 engine with 350 trans, 8" Ford rear end with a 2.70-78 (?) ratio, and 30" tall rear tires. I'm thinking of a 3.80 to 4.10 ratio for street/highway driving with those tall rear tires.

Question, what ratio should I use?

Question, what kind of limited slip unit should I use in the 8"?

Question, how hard is it correctly set up the R&P pattern? Should I farm this job out?

Thanks, John Palmer

With a 3.80 gear to run 70MPH you will be running right at 3,000 RPM, the same calculator sez 2500 RPM @ 70 MPH would be a 3.18 gear. You can pick a ratio from there. I've ran the Auburn posi units in some lighter cars with no problems, fairly sure they have one for an 8". As far as setting in up it's like any other rear end, if you don't have a pinion depth setting tool, probably should farm it out.

John,
I am afraid of lockers in a light car such as a duece. A broken axle means a very quick turn in the opposite direction. If you are not using the car for anything more than fun and crusin' an open differential is all you will ever need and an 8" is tough as well as being cheaper than a 9". A 9" = braggin' rights !! Tho I do have one in my latest - a '57 round back cover.

This is the list of Ford 8" diffs by door post code you can use - remember, pull the axles, drive shaft and nuts on the front of the case and it drops out like a 9", it only weighs less.

OPEN - LOCK - RATIO
2 - K - 2.75
3 - L - 2.79 (What you have)
6 - 0 - 3.00
7 - None - 3.40 (Good choice)
9 - R - 3.25
A - S - 3.50
G - None - 3.55
None - V - 3.91

I hope this looks OK - the preview showed it screwed up - the first letter/number is for open, the second for locker and the last is the ratio.

3.40 and 3.50's are in Granada's and Monarch's. My last 3.40 came from a 77 - 78 Pinto (??) wagon, tho the 4 lug axles were not useable. Standard shift Mavericks had 3.00 and 3.25. Some 6 cyl Torinos had 8"ers as well


Scout out your local junkie

IMO you do not want to run a Locker, but you definitely need to run a Posi. The Posi will give you the best streetable oppurtunity to get your car hooked up to the pavement.


Kitz

With a 350/350? In a 2500 pound car? No way is any type of locking/posi diff necessary or even desired unless the car is driven in snow or torrential rain. Anything but an open final drive will exacerbate bump steer and potentially lead to serious consequences if an axle were to break and this is an 8" he is talking about. While the 8" R&P itself is very strong, the axle shafts are less than great unless they are aftermarket, i.e. Currie

Originally Posted by Irelands child

With a 350/350? In a 2500 pound car? No way is any type of locking/posi diff necessary or even desired unless the car is driven in snow or torrential rain. Anything but an open final drive will exacerbate bump steer and potentially lead to serious consequences if an axle were to break and this is an 8" he is talking about. While the 8" R&P itself is very strong, the axle shafts are less than great unless they are aftermarket, i.e. Currie

Well, I've run posi's in a lot lighter cars without problems!!! A spool will do exactly as you said, but a posi unit in a 2500 pound car will work just fine, done it a few times.

Differ'nt strokes for diffen'nt folks - twould be a dull world without these opinions. We'll can both stand by our own opinions and be satisfied, i.e. Ford vs. Chevy, Budweiser vs. Guinness, etc, etc.

I totally agree Dave.

A 350/350 under 500 hp ain't going to break any axles either. Tell me the axle diameter and I'll give you the torque required to yield (twist) them and fracture them. If you rev'ed her way up and dumped it something in the tranny or rear end will let go but not the axles in my experience. Axles are a lot stronger than differential gear teeth for example.

I have heard others say that Posi's don't work with light cars but yet it is an undeniable fact that it is done all the time and meanwhile the claim remains IMO totally unfounded. Is there a technical paper or article on the subject somewhere?

Regards, Kitz

Thanks Dave for the list of codes for the axle ratios, very helpful.

I would be happy to find a 3.50 or lower gear ratio used center section at a Pick-a-Part. Even without a limited slip it would work until I could afford to step up to a new unit "with a posi". Sorry, but it's going to have some kind of a limited slip rear end in it. It's too embarrasing to do a "one legged" burn out when I'm showing off, LOL.

I have no reservations about using a locker or posi on the street. We drag race a 1600# car with 260 N/A horsepower and then run a 70 horse shot of Nitrous. We have a spool in the car and have broken our share of parts including custom axles without any serious probems.

I run a Detroit Locker in my Fairlane with 4.11's and 9" with no bad effects from weather or anything else. If anyone mentions the ratchety noise I just say the exhaust needs to be louder. You can buy an 8" with a Track Lock (clutch posi) from Maeracing on ebay for a lot less than a Currie and all new parts in it. It will lack the cachet of the Currie for the bragging rights after the smoke show but it does come with your choice of gears.